The present invention relates to a method of determining activation of exhaust gas ingredient-concentration sensors adapted for use in controlling the air-fuel ratio of a mixture supplied to an internal combustion engine, and more particularly to a method of this kind, which utilizes output from the sensor for determining the activation of same.
Exhaust gas ingredient-concentration sensors, for example, oxygen-concentration (O.sub.2) sensors in general can produce proper output voltages exactly commensurate with the concentration of oxygen contained in exhaust gases emitted from an internal combustion engine only while the O.sub.2 sensor is in an activated state. The output voltage is excessively high while the sensor is in an inactivated state. While the O.sub.2 sensor is activated, it supplies an output voltage equal to a reference voltage V.sub.REF when the air-fuel ratio of the mixture assumes a stoichiometric ratio, whereas it supplies an output voltage higher than the reference voltage V.sub.REF when the air-fuel ratio is richer than the stoichiometric ratio, and an output voltage lower than the reference voltage V.sub.REF when the air-fuel ratio is leaner than the stoichiometric ratio.
There is known a method of determining the activation of an O.sub.2 sensor, which utilizes the above-mentioned output voltage characteristic of the O.sub.2 sensor in such a manner that it is determined that the O.sub.2 sensor is in an activated state when the output voltage of the O.sub.2 sensor is lower than a predetermined value V.sub.X1 during a predetermined decelerating condition of the engine in which the air-fuel ratio of the mixture supplied to the engine (hereinafter merely referred to as "the air-fuel ratio") is to be controlled to a leaner value than the stoichiometric ratio.
Another activation determining method has been proposed, e.g., from Japanese Provisional Patent Publication (Kokai) No. 62-162955, which is improved over the above known method such that it is able to always determine whether or not the O.sub.2 sensor is activated, irrespective of values of the air-fuel ratio, by supplementing secondary air into the exhaust pipe at an upstream of the O.sub.2 sensor to lean the exhaust gases supplied to the O.sub.2 sensor, during engine operation in which the air-fuel ratio is to be controlled to a richer value than the stoichiometric ratio, and then effecting determination of activation of the O.sub.2 sensor.
These known methods, however, had the disadvantage that accurate determination of activation of the O.sub.2 sensor is impossible to effect during deceleration of the engine. To be specific, in the case where the above-mentioned predetermined value V.sub.X1 is set at a value lower than the reference voltage V.sub.REF corresponding to the stoichiometric ratio, accurate determination of the O.sub.2 sensor activation is impossible to effect unless the actual air-fuel ratio is positively leaned. However, in the known methods, leaning of the air-fuel ratio is carried out by multiplying a basic value of a fuel supply amount, which has been read from a table of basic values previously set for various operating conditions of the engine, by a mixture-leaning coefficient which has a fixed value. Therefore, in the case where a basic value is read, which gives a richer air-fuel ratio than a theoretically proper value due to manufacturing variations between individual control systems or parameter detecting systems, aging in these systems, etc., it is impossible to positively lean the air-fuel ratio by correcting the read basic value by the mixture-leaning coefficient, thereby failing to effect accurate determination of the activation of the O.sub.2 sensor. Further, when the engine shifts into a decelerating condition, fuel adhering to an inner wall of the intake pipe is entrained into engine cylinders together with fuel being normally supplied through a fuel supply system, e.g., fuel injection valves so that there temporarily occurs enrichment of the air-fuel ratio. This also makes it impossible to effect accurate determination of the activation of the O.sub.2 sensor.